Makarand Paranjape

Electron-hole transport and photovoltaic effect in gated MoS2 Schottky junctions

Semiconducting molybdenum disulfphide has emerged as an attractive material for novel nanoscale optoelectronic devices due to its reduced dimensionality and large direct bandgap. Since optoelectronic devices require electron-hole generation/recombination, it is important to be able to fabricate ambipolar transistors to investigate charge transport both in the conduction band and in the valence band. Although n-type transistor operation for single-layer and few-layer MoS2 with gold source and drain contacts was recently demonstrated, transport in the valence band has been elusive for solid-state devices. Here we show that a multi-layer MoS2 channel can be hole-doped by palladium contacts, yielding MoS2 p-type transistors. When two different materials are used for the source and drain contacts, for example hole-doping Pd and electron-doping Au, the Schottky junctions formed at the MoS2 contacts produce a clear photovoltaic effect.

Dry etching of polydimethylsiloxane for microfluidic systems

A fluorine-based reactive ion etch (RIE) process has been developed to anisotropically dry etch the silicone elastomer polydimethylsiloxane (PDMS). This technique complements the standard molding procedure that makes use of forms made of thick SU-8 photoresist to produce features in the PDMS. Total gas pressure and the ratio of O2 to CF4 were varied to optimize etch rate. The RIE recipe developed in this study uses a 1:3 mixture of O2 to CF4 gas resulting in a highly directional and stable etch rate of approximately 20 μm per hour. Selective dry etching can be performed through a photolithographically patterned metal etch mask providing greater precision and alignment with preexisting molded features. The dry etch process is presented in this article along with a brief comparison to recently reported wet etch approaches.

Mechanism of NO2 detection in carbon nanotube field effect transistor chemical sensors

We report an experimental method that clearly determines the sensing mechanism of carbon-nanotube field effect transistors. The nanotube/electrode contacts are covered with a thick and long passivation layer that hinders their exposure to chemicals in a controlled fashion, leaving only the midsection of the nanotube exposed. In the case of nitrogen dioxide, a considerably delayed response is fully consistent with the diffusion of the gas through the passivation layer. The results clearly indicate that nitrogen dioxide detection is due to changes at the interfaces between the nanotube and the electrodes and not to molecules adsorbed on the nanotube surface.

Wafer-level assembly of carbon nanotube networks using dielectrophoresis

We use dielectrophoresis (DEP) to controllably and simultaneously assemble multiple carbon nanotube (CNT) networks at the wafer level. By an appropriate choice of electrode dimensions and geometry, an electric field is generated that captures CNTs from a sizable volume of suspension, resulting in good CNT network uniformity and alignment. During the DEP process, the electrical characteristics of the CNT network are measured and correlated with the network morphology. These experiments give novel insight into the physics of DEP assembly of CNT networks, and demonstrate the scalability of DEP for future device applications.

A PDMS dermal patch for non-intrusive transdermal glucose sensing

Abstract This paper describes the fabrication of an adhesive bandage comprised of multiple, compliant polydimethylsiloxane (PDMS) micro-fluidic elements to perform controlled and non-intrusive transdermal sampling of glucose, or any other bio-molecule present in interstitial fluids. The patch-like device, to be worn on the skin, has PDMS component layers that form vertically oriented micro-fluidic channels and reservoirs. In addition, micro-heater elements are integrated onto the PDMS layer that will be in contact with the skin, and are used to thermally ablate tiny micro-pores through only the dead-skin layer, allowing for easier diffusion of normally trapped bio-molecules, such as glucose, to the skin surface. The dermal patch, containing micro-channels and fluid-encapsulated reservoirs, assist in the transport of glucose molecules from just beneath the dead-skin layer to a colorimetric detection membrane situated on top of the bi-layer PDMS patch. This paper will focus on the fabrication of the prototype PDMS patch and the challenges encountered during wafer-scale batch production. Preliminary in vitro studies using human graft skin samples are included to illustrate the non-inflammatory micro-ablation procedure.

Dry release of polymer structures with anti-sticking layer

A dry release method using a thin Teflon™ layer for SU-8 multilayered polymeric microstructures is presented. The low surface energy of Teflon makes the adhesion of SU-8 and substrate poor, enabling the SU-8 polymer photoresist to be removed after the devices have been fully processed. The surface energy was measured using the open-crack method, and the surface roughness and deformation of the released SU-8 were minimized in our processing. The dry release technique eliminates the diffusion limited problem in wet etching and is suitable to package complex three-dimensional polymer microfluidic devices. One such example, which provided the original impetus to formulate a dry release process, is a multilayered SU-8 structure that encapsulates small quantities of fluid. This device is being developed for a biomedical application, and will be used throughout this article as an example of a complex SU-8 structure that uses the dry release process.

A photolithographic process for fabrication of devices with isolated single-walled carbon nanotubes

We have successfully fabricated devices with isolated single-walled carbon nanotubes (SWNTs) using exclusively standard i-line (365 nm) photolithography. Catalyst islands were patterned with an SU-8-photoresist-based process. This method provides well-defined islands, down to 1 µm in size. The islands are clearly visible with an optical microscope and are used as alignment marks for optical alignment of subsequent layers. SWNTs were grown by chemical vapour deposition (CVD). Contacts to the nanotubes were fabricated by standard photolithography.

A simple deflection-testing method to determine Poisson's ratio for MEMS applications

A method to determine Poissons ratio of thin films employing a simple experimental deflection test and a simulation program has been developed. According to the method, knowing Youngs modulus and measuring the deflection at an arbitrary point on the thin film, the Poissons ratio can be determined using any mechanical finite element analysis (FEA) program. Two examples are considered to test the method, with both yielding the same results. Due to the relative simplicity involved with this procedure, it is believed that the method can be used by MEMS researchers to determine Poissons ratios of their own thin films of interest. In addition, in the case that Poissons ratio is known and Youngs modulus is unknown, the method may be applied to find the Youngs modulus.

Thermal ablation of PMMA for water release using a microheater

A new approach was developed in this work to ablate a micropore through a polymethylmethacrylate (PMMA) layer using a microheater for the on-demand release of water stored in a microreservoir. The size of the ablated micropore in the PMMA capping layer is mainly governed by the heater temperature profile and ablation time. Furthermore, the molten PMMA resulting from the heating energy tends to retract away from the micropore location, taking away the gold heater lines from the pore area. This prevents the possibility of any gold traces blocking the flow of water released from within the microreservoir. Simulation was conducted to find temperature profiles on the surface of the microheater, and the results were used to interpret the phenomena observed in the ablation process. Simulation was also conducted for the case when the microreservoir was filled with water. In addition, two alternative ablation materials, unexposed SU-8 and polydimethylsiloxane (PDMS), were examined as possible microreservoir capping layers. The approach developed has potential applications in microfluidic systems to release encapsulated fluids in a controllable manner.

Microstructures etched in doped TMAH solutions

Tetra-methyl ammonium hydroxide, or TMAH, is an anisotropic silicon etchant that is gaining more and more attention in the fabrication process of mechanical microstructures and device isolation, as an alternative to the more usual KOH and EDP etchants [1]: because of its high compatibility with conventional IC processes, due to the absence of metal ions in it. The possibility to passivate the aluminum metalisation in properly saturated TMAH solution has also been demonstrated by doping the solution with appropriate amounts of silicon or silicic acid [2, 3]. This increases the range of application of these etchants, simplifying both the post processing and the etch set-up configuration. In this paper we present some different technological solutions adopted for the fabrication of 3D structures using as anisotropic etchant doped TMAH solutions. The effects of the additives on etch uniformity and surface roughness are studied. Using the etching results under different doping conditions of the TMAH solutions, we obtained silicon bulk-micromachined structures with controlled surface roughness. Furthermore the aluminium passivation permits to etch devices with no protection of the metalisation layers as some applications require (i.e. bolometers).